1. Technical Field
The present disclosure is directed to systems and methods that utilize agglomerates of nanoparticles to effect advantageous filtration. In exemplary embodiments of the present disclosure, agglomerates of nanoparticles are used as a HEPA filtration system to remove solid or liquid submicron-sized particles, e.g., MPPS, in an efficient and efficacious manner.
2. Background Art
Air filters with particularly high collection efficiency for submicron size particles (under 1 μm) are generally referred to as High Efficiency Particulate Air (HEPA) filters. Submicron size particles are generally the most difficult to filter and are commonly known as the “most penetrating particle size” (i.e., MPPS). HEPA filters are used extensively in the microelectronics field (e.g., in clean rooms) and in the pharmaceutical industry. HEPA filters are also used in hospitals, in food and cosmetic production facilities, and even in residential settings, e.g., in air purifiers and vacuum cleaners. In each of these applications, the filtration objective is either to prevent contamination of a particularly sensitive product with particulate pollutants or to protect human beings from dangerous particulates, such as microorganisms (e.g., bacteria, viruses and/or mold), pollen, asbestos, etc.
HEPA filters are generally fiber-based and are made up of an entanglement of thin (usually less than one micron in diameter) fibers. A scanning electron microscope (SEM) image of a conventional fiber-based filter is presented in FIG. 1. With fiber-based materials, particles are collected by several classical mechanisms, such as diffusion, interception and inertial impaction. Two important performance-related parameters associated with these filters are pressure drop and collection efficiency. Collection efficiency (E) is related to penetration (P) by the formula: P=1−E. These performance-related parameters generally depend on the filter structure (e.g., packing density, fiber diameter), operating conditions (e.g., filter velocity, temperature) and the properties of the systems (e.g., aerosols) to be filtered (e.g., density, mean particle size, particle size distribution, solid or liquid). In addition, the performance-related parameters depend strongly on filter loading.
SEM studies of the filter loading of HEPA fiber filters with solid particles show that the filtration initially takes place in the depth of the filter with the formation of chain-like agglomerates called dendrites. During this initial stage in the filtration process and at constant face velocity, the pressure drop across the filter generally rises linearly with the amount of mass/particles collected. However, as the dendrites begin to fill the spaces between the fibers of the filter, a filter cake of increasing thickness begins to form at the upstream surface of the filter and the slope of the pressure drop with increasing loading rises sharply, indicating that the filter is being clogged.
For liquid particles (mists), in the early stage of filtration using a fiber-based HEPA filter, particles are deposited as droplets around the fibers and the pressure drop rises slowly with mass collected per unit of filter area. However, at a certain point during filtration, a sharp exponential rise in pressure drop is observed. This behavior may be attributed to the presence of a liquid film covering the filter surface. It is believed that droplets deposited on the filters progressively grow and join together to form bridges at the intersection of several fibers. At the point of clogging, all (or substantially all) of the interstices of the first layer of fibers are filled in, forming a film covering the filter surface. It is noted that clogging occurs at a much higher loading level for liquid particles, e.g., mists, than for solid particles.
When clean, HEPA fiber-based filters provide excellent filtration efficiency and low pressure drop for both solid and liquid MPPS and filtration occurs throughout the depth (deep bed filtration) of the filter. However, as soon as the upstream surface becomes heavily clogged with particulates, filtration only occurs at the filter's surface (cake filtration) leading to a sharp rise in pressure drop. Based on this sharp rise in pressure drop, filtration performance becomes unacceptable and the filter needs to be cleaned or replaced. In typical fiber-based HEPA filters, this degradation in performance occurs at a loading (mass of particulates collected) of about 1-7 g/m2 of filter area.
Accordingly, a need exists for improved filtration systems and methods. More particularly, a need exists for filtration systems and methods that offer efficient and reliable filtration for sub-micron size particles, e.g., MPPS. The foregoing needs extend across a host of fields, industries and applications, including, for example, the microelectronics field, the medical device/pharmaceutical industries, health care applications, including hospitals, food and cosmetic production facilities, and residential settings, e.g., in air purifiers and vacuum cleaners. Thus, a need exists for filtration systems and methods that effectively filter MPPS from feed streams that contain liquid and/or solid particles so as to prevent/reduce the potential for contamination and/or to protect systems/individuals from undesirable particulates, e.g., microorganisms such as bacteria, viruses and/or mold, pollen, asbestos, and the like.
These and other needs are satisfied by the systems and methods of the present disclosure, as will be apparent from the description which follows, particularly when read in conjunction with the figures appended hereto.